Method of increasing the yield strength of cold formed steel sections

ABSTRACT

Yield strength of a cold rolled steel section is increased and controlled by performing a predetermined amount of strain by way of cold working in an in-line roll forming process followed by a controlled amount of strain aging wherein the temperature of the steel section is elevated to a point below 500° C. and held at an elevated temperature for a time up to 30 seconds. The heating typically takes place by induction heaters (16) and the time aging may be provided in an in-line galvanizing bath (17) before cooling the steel in a quench bath (18). The effect is further enhanced by further cold working and the consequent additional strain in forming rolls (19). For a given steel composition the degree of yield enhancement can be controlled by the temperature and tie parameters and also by the degree of initial roll forming in shaping rolls (10).

TECHNICAL FIELD

This invention relates to a method of increasing the yield strength ofcold formed steel sections during the course of in-line roll forming asteel strip to a desired structural shape.

BACKGROUND ART

The process of forming steel strip into desired structural shapes suchas rectangular hollow section, circular tube, angles, channels and otheropen profile sections is well known and has been in use for many years.The feed material is normally so-called "black" steel which hastypically been formed into steel strip by a hot rolling process in amanufacturing mill.

In the past one normally recognised method of obtaining increased yieldstrength of the finished product formed from a subsequent cold rollingprocess, is to alter the "chemistry" of the steel strip, i.e. by addingvarious alloying metals into the composition of the steel before hotrolling. Another method is the use of thermomechanical practice duringhot rolling. These are expensive processes due to the cost of the metalalloy and the process for obtaining the desired mix of alloy, thetechnological cost of processing by thermomechanical practices, and alsobecause of the necessity to keep inventory of different types of metalsection in order to meet the demand for different performancecharacteristics at an economical price.

For these reasons, the vast majority of all cold rolled steel sectionsare formed from common black steel with the size and weight of thesection simply being increased where desired to obtain the necessaryload-bearing characteristics.

There are however many applications where it is desirable from bothengineering and economic points of view to enhance the yieldcharacteristics of the steel from which a structural section is formedin order to give increased performance compared with a similar sectionrolled from black steel in the conventional manner.

DISCLOSURE OF INVENTION

The present invention therefore provides a method of increasing theyield strength of cold rolled steel sections as part of an in-linemanufacturing process, comprising the steps of passing a steel sectionwhich has been at least partially cold worked and thereby subjected to apredetermined amount of strain, through a heating stage wherein thetemperature of the steel section is elevated to a range between 200° C.and 500° C., and holding the temperature of the steel section in thattemperature range for a time range between two and thirty seconds, thetemperature and time combination being selected within the said rangesto achieve a predetermined degree of strain ageing.

Preferably the method includes steps of cooling the steel section afterheating and strain ageing and then performing subsequent cold working onthe steel section.

Preferably the step of passing the steel section through a heating stagecomprises heating the steel section to a temperature between 200 and450° C. over a time between two and thirty seconds and holding thetemperature at at least 440° C. for between one and fifteen seconds.

More preferably the step or passing the steel section through a heatingstage comprises heating the steel section to a temperature between 350and 400° C. over a time between two and ten seconds and holding thetemperature between 440 and 460° C. for between two and six seconds.

The step of cooling the steel section reduces the temperature of thesection to below 90° C. and preferably to between 25 and 45° C. beforesubsequent cold working.

In one form of the invention the steps of elevating the temperature andholding that elevated temperature are performed by the preheating andsubsequent coating of the steel section in an in-line galvanisingoperation.

Preferably the steel section has a steel composition containing between0.01 and 0.25% carbon and between 0.001 and 0.006% nitrogen.

BRIEF DESCRIPTION OF DRAWINGS

Notwithstanding any other forms that may fall within its scope, onepreferred form of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a line diagram of a mill for the continuous forming by coldworking of heavy gauge hollow sections from steel strip; and

FIG. 2 is a graph of the temperature of a steel section passing throughthe mill shown in FIG. 1.

MODES FOR CARRYING OUT THE INVENTION

In one form of the invention as will now be described the heating of thesteel strip after initial cold working is performed as part of anin-line galvanising process although it will be appreciated that theheating could be performed independently of galvanising on a plain blacksteel section.

The cold working mill shown in the attached drawing takes coils of hotrolled steel strip 1 which are placed in a coil feed magazine 2 beforethe strip is unrolled and passed through an uncoiling station 3, pinchrolls 4 and leveller rollers 5 to flatten the strip and remove any coilset. The strip then passes through a splice welding station 6 wheresubsequent coils are joined end-to-end to form a continuous feed stripfor the mill.

The strip is then pulled by pinch rolls 7 into an accumulation system 8and then fed through a shot blast station 9 to prepare the surface ofthe steel strip.

The initial roll forming of the strip is performed in the shapepreparation machine 10 where the initial cold working takes place as thesteel section is deformed to its initial configuration at approximatelyatmospheric temperature and, where it is desired to form a hollowsection, longitudinal edge welding of the strip takes place.

The steel section 11 then passes into a cooling section 12 to cool themetal after the welding operation.

Where it is desired to provide an in-line coating, e.g. a galvanisedcoating of the section, the section then passes through an acid picklingstage 13 and a rinsing stage 14 with wiping of the surface beingeffected after each stage by air knives 15 to remove excess liquid.

The section then passes into heating apparatus 16 which may be by anysuitable form but is preferably conducted by electric induction heating.This may be carried out in an inert gas atmosphere in order to preservethe surface condition of the steel section. The induction heating phaseraises the temperature of the section to between 200 and 450° C. over atime period between two and thirty seconds. In the preferred form of theinvention the induction heating raises the temperature to between 350and 400° C. over an exposure time of between two and six seconds.

The heated section then passes rapidly into an in-line galvanising stage17 where, as part of the galvanising process, the temperature of thesection is held between 440° C. and 460° C. for between one and fifteenseconds. In the most preferred form of the invention the temperature inthe galvanising stage is held between 445° C. and 455° C. for betweentwo and six seconds.

The section then passes through a quenching station 18 where thetemperature of the section is reduced to between 25 and 45° C.

These temperature profiles can be clearly seen in FIG. 2 where thenumbers in the boxes at the foot of the graph relate to the differentstages in the roll forming process shown in FIG. 1 and are designated bysimilar numbers and wherein the temperature rise in the induction heater16 is shown at 26 and the temperature holding profile in the galvanisingbath at 27. The quenching taking place at 18 results in the temperatureprofile 28. By way of comparison, the normal cold roll forming processfor black steel which is not galvanised can be seen at 29.

Subsequent final forming by cold working is then performed by theforming rolls 19 before the section passes through a rinsing station 20and a coating station 21 where the section may be dried by air knives 22and a final coating, e.g. of clear polymer may be applied.

Finally the section passes through a drying station 23 to a flying saw24 where it is cut into desired lengths and passed to an unloadingstation 25.

By elevating the temperature of the section between the initial coldworking in the shape preparation machine 10 and the final forming rolls19, a "strain aging" operation is performed on the steel section whichconsiderably enhances the yield strength and the ultimate tensilestrength of the product compared with cold formed steel sections whichare not heated between the initial and final cold rolling operation. Forcontinuously cast Al-Si killed 1015 type steels, this increase instrength is typically 55 MPa for the yield strength and 50 MPa for theultimate tensile strength. For continuously cast Al--Si killed 1006 typesteels, this increase in strength is typically 30 MPa for the yieldstrength and 30 MPa for the ultimate tensile strength. The degree ofstrength enhancement depends on the amount of cold working occurring inthe initial and final forming operation, the temperature and duration ofthe heating in-stages 16 and 17 and the chemical composition of thesteel, particularly the carbon content.

The degree of strength enhancement can therefore be tailored to anydesired end product either by controlling the parameters of the heatingand strain ageing process as set forth above or more particularly bycontrolling the amount of cold working occurring in the initialoperation, i.e. typically in the shape-forming rolls 10. A certainamount of inherent strain will occur in preforming the base steel stripto the desired shape before galvanising but if this is insufficient toachieve the desired amount of yield or strength enhancement, an"artificial" degree of strain may be added at this point. This may beachieved either by longitudinal working of the metal strip, e.g. to acurved profile and then back to a flat profile or by lateral working bypassing the flat steel strip in an "S" profile or similar, i.e. througha sinusoidal path or between pairs of bridal rolls. As the strain ageingprocess builds upon the strain induced by the initial cold working it istherefore possible to tailor the ultimate yield characteristics of thefinished product by controlling the amount of initial strain in thismanner.

The chemical composition of the steel and in particular the carboncomposition have also been found to have a significant effect on thedegree of yield enhancement relating from the initial strain andsubsequent strain ageing. The effect has been found to be applicableover carbon ranges between 0.01% and 0.25% carbon in the steel andnitrogen ranges between 0.0015% and 0.0045%. Particularly advantageousresults have been achieved with carbon contents in the 0.04% to 0.17%ranges. The effect has been found to be equally applicable to hot rolledstrip and standard general purpose cold rolled strip base materials withcarbon and nitrogen contents in these ranges.

Although the preferred form of the invention has been described as oneincorporating an in-line galvanising station 17, the increased yieldstrength effect is independent of whether the section is galvanised ornot as it is the heating in stages 16 and 17 which contributes to thestrain aging of the steel section. It is of course possible to omit thegalvanising station 17 and simply to heat the black steel section in theheating stage 16 and hold it over the defined temperature range for thedefined time in order to obtain the increased strength properties of thesteel section.

We claim:
 1. A method of increasing the yield strength of cold rolledsteel sections as part of an in-line manufacturing process, comprisingthe steps of passing a steel section which has been at least partiallycold worked and thereby subjected to strain, through a heating stagewherein the temperature of the steel section is elevated to a rangebetween 200° C. and 500° C., and holding the temperature of the steelsection in that temperature range for a time range between two andthirty seconds, the temperature and time combination being selectedwithin the said ranges to achieve a predetermined degree of strainageing.
 2. A method as claimed in claim 1 including the steps of coolingthe steel section after heating and strain ageing and then performingsubsequent cold working on the steel section.
 3. A method as claimed inclaim 1 wherein the temperature of the steel section is elevated to arange between 200 and 450° C. over a time between two and thirty secondsand wherein the temperature of the steel section is then held at atleast 440° C. in a time range between one and fifteen seconds.
 4. Amethod as claimed in claim 1 wherein the temperature of the steelsection is elevated to a range between 350 and 400° C. over a timebetween two and ten seconds and wherein the temperature of the steelsection is then held at between 440° C. and 460° C. in a time rangebetween two and six seconds.
 5. A method as claimed in claim 2 whereinthe step of cooling the steel section reduces the temperature of thesection to below 90° C. before subsequent cold working.
 6. A method asclaimed in claim 5 wherein the step of cooling the steel section reducesthe temperature of the section to between 25° C. and 45° C. beforesubsequent cold working.
 7. A method as claimed in claim 1 wherein thesteps of elevating the temperature and holding that elevated temperatureare performed during the preheating and subsequent coating of the steelsection in an in-line galvanising operation.
 8. A method as claimed inclaim 1 wherein the steel section has a steel composition containingbetween 0.01 and 0.25% carbon.
 9. A method as claimed in claim 1 whereinthe steel section has a steel composition containing between 0.001 and0.006% nitrogen.